Thin films composed of Mefp-1 and ceria nanoparticles have shown an increasing corrosion inhibitioneffect with time for carbon steel in acidic aqueous solutions containing phosphate, which motivates adetailed study of the inhibition mechanism by in situ confocal Raman micro-spectroscopy (CRM) andelectrochemical impedance spectroscopy (EIS) measurements. The presence of both CeO2and ferricoxides in the thin composite film was demonstrated by X-ray photoelectron spectroscopy analysis. TheRaman spectra assisted by DFT calculations suggest that Mefp-1 forms tri-Fe3+/Mefp-1 complexes andbinds to ceria nanoparticles in the composite film. The in situ CRM measurement allow us to followthe development of corrosion products. The measurements show a mixture of Fe oxides/oxyhydroxides,and also indicate that ferrous oxides may be further oxidized by the composite film. Moreover, phos-phate ions react with the Fe ions released from the surface to form iron-phosphate deposits, whichbecome incorporated into the corrosion product layer and the composite film. The EIS measurementssuggest a layered surface structure formed by the initial Mefp-1/ceria composite layer and the corrosionproducts/iron-phosphate deposits. These measurements also demonstrate the greatly increased inhibi-tion effect of the composite film in the presence of the phosphate ions. The consistent CRM and EIS resultssuggest that the iron-phosphate deposits heal defects in the composite film and corrosion product layer,which results in a significantly improved corrosion inhibition of the Mefp-1/ceria composite film duringinitial and long term exposure.